This invention relates to a blind rivet.
Blind rivets are well-known and generally comprise a mandrel having a pulling stem with a radially enlarged head attached at one end and a tubular shell having a generally cylindrical shaped shank with a radial flange formed at one end of the end of the shank for engaging a face of the workpiece. An axial bore is formed through the length of the shank and the flange. Part of the pulling stem is located within the bore, the enlarged head abutting against the end of the shank remote from the flange. The rest of the pulling stem extends beyond the flange away from the tubular shell. A portion of the shell is of reduced section or a breakneck which is formed on part of the stem which is located within the shank. When the blind rivet is set, the flange is held stationary whilst the exposed part of the pulling stem is pulled axially away from the flange so that the enlarged head is forced to pass through the bore. Because the diameter of the enlarged head is substantially larger than the bore, it causes the shank to deform to form an annular bulge or fold which projects radially outwardly from the shank. Thus the radially outward bulge forms the blind head and engages the opposite side of the workpiece to that of the flange. Once the pulling force on the stem exceeds a predetermined amount, the breakneck breaks leaving the rivet set. The rest of the stem can then be removed and discarded.
One type of blind rivet comprises a tubular shank of uniform outer diameter and a mandrel which comprises four indentations located immediately below the head of the mandrel which form a cruciform in shape when viewed in cross-section. It is into these indentations that the body material of the shell is displaced when the mandrel is assembled and consolidated to the tubular shank which results in the rivet having a shank greater thickness around these indentations and of higher material hardness. The outer diameter of the shank remains uniform even at the section which surrounds the indentations.
However, due to variations in the manufacture of the tubular shank and in the subsequent assembly and consolidation to give the thickened end, small gaps between the inner wall of the shank and the indentations can remain. Also, the mandrel may be eccentrically located within the shank and thus the material of one side of a rivet body may not fully fill the indentations beneath the mandrel head.
As described above, during the setting of blind rivets the mandrel is subjected to a tensile loading. Just prior to when the mandrel breaks at the breakneck, the tensile loading will be at its maximum. As the breakneck breaks, the tension or strain energy within the part of the mandrel retained within the body of the rivet is released. This can cause the mandrel head to dislodge from the position where the mandrel stem breaks and result in a gap between the indentations in the mandrel and the thickened portion of the rivet body through which water can pass.
Accordingly, there is provided a blind rivet comprising:
a body having a generally cylindrical shank, a radial flange formed at one end of the shank and a bore which extends axially through the flange and through at least part of the length of the shank; and
a mandrel comprising a mandrel head located adjacent end of a bore remote from the flange, a stem attached to the mandrel head which passes through the length of the bore and extends beyond the flange and which comprises a breakneck formed on the stem, wherein the mandrel comprises a plurality of indentations formed around the stem adjacent to the mandrel head and which taper partway along the stem away from the mandrel head characterised in that the shape of the cross-section of the stem at the point where the indentations are located is substantially square.
By designing the blind rivet in this manner, it prevents or at least reduces the effect of the recoil of the mandrel head portion on setting and prevents, or at least reduces, the separation of the mandrel and the thickened portion of the shank and thus prevents or reduces the amount of ingress of water or moisture through the rivet body. Furthermore, such a design can improve the mandrel retention in the rivet body prior to setting and can prevent the portion of the mandrel retained within the body of the rivet after setting from moving thus resisting the potential for moisture ingress.
The shank can have a thickened end at the region of indentations so that the stem is prevented from movement by virtue of the reverse taper portions thus further improving the mandrel retention in the rivet body.
Further tapers can be provided just prior to the breakneck to prevent the mandrel digging into the walls of the bore as the mandrel passes through the bore when the rivet is being set. As the mandrel does not dig into the body of a rivet, the setting of a rivet is not hindered or prevented.
Ideally the outer diameter of the shank has a uniform diameter. Preferably the material of the shank completely fills the indentations. This results in the thickness of the wall of the shank around the stem of the point where the indentations are formed being much thicker and can also be of higher hardness.